Improving the non-radical/radical transformation efficiency of halogenated polycyclic aromatic hydrocarbons by using metal ion-exchanged cryptomelane-type OMS-2 under moisture-unsaturated conditions

Halogenated polycyclic aromatic hydrocarbons (HPAHs) have recently remerged as major soil contaminants. However, limited studies have focused on the transformation and removal of HPAHs in soils. The transformation behaviors of HPAHs on cryptomelane-type manganese oxide (K(I)-OMS-2) and its metal ion-exchanged M-OMS-2 (M = Fe(III), Cu(II), and Zn(II)) under moisture-unsaturated conditions were systematically investigated. The transformation kinetics results showed that the transformation rate constants (k) of HPAHs were 0.077–0.829, 0.071–0.629, 0.068–0.530, and 0.064–0.259 d−1 for Fe(III)-OMS-2, Cu(II)-OMS-2, Zn(II)-OMS-2, and K(I)-OMS-2. The metal ion exchange treatment of K(I)-OMS-2 enhanced the transformation efficiency of HPAHs. The characterization (H2-TPR, CV, ESI, XPS, and EPR) and theoretical calculation (density of states and formation energy of oxygen vacancies) results indicated that the non-radical/radical transformation efficiency of HPAHs on M-OMS-2 was higher than that on K(I)-OMS-2. The transformation rate of HPAHs was positively correlated with the redox potential of metal ions and superoxide radical yield of M-OMS-2. Additionally, this study revealed that HPAHs with lower redox potential and more strongly bound to M-OMS-2 were more preferentially transformed. Finally, the experimental results of the addition of M-OMS-2 to the natural soil revealed that M-OMS-2 aids the transformation of HPAHs in soils. This study reveals the chemical transformation behavior of HPAHs in soils and suggests that M-OMS-2 can a promising agent for soil remediation.